Fixes several bugs in code generation, including handling of booleans,
comparisons and shifts. The tests that get through code generation now
run successfully (except for the tests that are known to fail on
https://wasm-stat.us/).

This change also includes improvements to the test infrastructure. The
wasm test runner has a list of expected failures to skip. The tests
now run in parallel, which significantly cuts down the time to run the
whole test suite.

Finally, there are some minor improvements to the WASM runtime,
including an implementation of syscall20, i.e. getpid().

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4369
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1900213002 .